Nagamitsu Yoshimura

Outgassing characteristics and microstructure of an electropolished stainless steel surface

Outgassing characteristics of an electropolished stainless steel (304) pipe wall were investigated by an isolation method. The free outgassing rate after an in situ bakeout (150 °C, 20 h) was estimated as low as 1.6×10−12 Pa l s−1cm−2. After the in situ bakeout, H2 molecules were steadily evolved from the pipe wall, whereas most of CO, C, CH4, and CO2 molecules were emitted from the operating mass spectrometer and Bayard–Alpert gauge with incandescent filaments. Auger depth profile analysis revealed that the oxide layer of an electropolished surface was cleaner, thinner, and finer in microstructure than that of a buff‐polished surface. This is the reason why an electropolished surface showed a very low outgassing rate.

Outgassing characteristics and microstructure of a ‘‘vacuum fired’’ (1050 °C) stainless steel surface

‘‘Vacuum fired’’ (1050 °C) type 304 stainless steel (SS304) surfaces were investigated with scanning electron microscopy and Auger electron spectroscopy. The grain boundaries of the surface were vague and shallow, which occurred due to elemental diffusion at high temperature in vacuum. The newly formed oxide layer of the vacuum fired surface was much thinner, and could be said to be finer in microstructure than the native layer of an ‘‘as‐received’’ surface. A vacuum fired (1050 °C) SS304 chamber was evacuated by a sputter ion pump whose vessel was pretreated by vacuum firing (1050 °C). An extremely high vacuum of 1.5×10−9 Pa (N2 equivalent pressure) was indicated by an extractor ionization gauge after a mild bake (170 °C) following an air exposure. The outgassing rate of the chamber wall at the elapsed time of one day after a mild bake (170 °C) was roughly estimated as low as 2×10−11 Pa l/s cm−2 (N2 equivalent value) by an orifice method. Vacuum firing has the effects of degassing the gas molecules solut...

Matrix calculation of pressures in high‐vacuum systems

A matrix analysis method has been applied to two high‐vacuum systems: an outgassing pipe and an electron microscope. These vacuum systems were analyzed as linear vacuum circuits with pressure sources, current sources, and conductances by using a digital computer. The pressures at six points along an outgassing pipe were calculated for two different models. A 5% difference of pressures was noted between the models. Pressures throughout an electron microscope high‐vacuum system were also successfully calculated by a computer. The pressure varied over three orders of magnitude depending on position.

A differential pressure‐rise method for measuring the net outgassing rates of a solid material and for estimating its characteristic values as a gas source

A differential pressure‐rise method for measuring net outgassing rates of solid materials is introduced, which has an outstanding advantage in that the error due to chamber walls and the vacuum gauge is much reduced. The differential method was successfully applied to measuring net rates of solid materials. The net rate K per unit surface area of a solid material at a pressure P could be practically expressed as K=K0(1−P/Px), where the characteristic values Px and K0 are estimated by measuring two net rates at two different pressures.

Resistor network simulation method for a vacuum system in a molecular flow region

A simulation method to obtain the pressure distribution in a complex vacuum system has been proposed. The method introduces a new concept with regard to the function of each component of the vacuum system. The vacuum pump is regarded as a ‘‘vacuum resistor’’ connected at one side to the perfect vacuum. The gas source is regarded as a pressure generator connected to the vacuum. The conducting pipe is regarded as a vacuum resistor between the above elements. The vacuum sides of the pump element and the gas source element are assumed to be connected together by an imaginary route, and thus the vacuum system can be regarded as a closed vacuum circuit network. Such a vacuum circuit network may be replaced by an electric circuit network of a simulator for the vacuum system. The simulator was employed for the high vacuum system of an electron microscope in order to obtain the pressure distribution. The results obtained were in good agreement with the actually measured pressure distribution.

Microdischarges on an electron gun under high vacuum

Two types of microdischarges on an electron gun operated at 100 kV—one of which occurs between the high‐voltage electrodes and the other over the insulator surface—were experimentally examined, by concentrating the greatest interest on gas molecules on the respective surfaces. Microdischarges between the high‐voltage electrodes in high vacuum were induced by gas molecules adsorbed on the electrode surfaces. Argon glow cleaning showed a distinct conditioning effect on the electrode surfaces. Microdischarges over the insulator surface were enhanced by the outgassing from the insulator. The reason is that the insulator surface with a high density of gas molecules causes high yield secondary electron emission, leading to positive charge‐up over the insulator surface. Thermal degassing for the electrodes and insulator showed a conditioning effect to reduce microdischarges between the electrodes and over the insulator surface.

Modeling of outgassing or pumping functions of the constituent elements such as chamber walls and high-vacuum pumps

Abstract Chamber walls, subjected to “in situ” baking, sometimes show a pumping function for a high vacuum, while a high-vacuum pump sometimes shows outgassing in an ultrahigh vacuum. Such functions of the system elements can be represented by the internal pressures P X . All the system elements, such as chamber walls, pumps, and pin holes through a pipe wall, can be replaced by a pressure generator with the internal pressure P X and the internal flow impedance R X in the equivalent vacuum circuit. The internal pressure P X of the chamber wall varies depending on the wall history under high vacuum. The equivalent vacuum circuit composed of many characteristic values ( P X , R X ) and flow impedances R can represent the gas flows in the original high-vacuum system.

Two‐point pressure method for measuring the outgassing rate

The two‐point pressure method for measuring the outgassing rate of a solid material has been introduced, in which the pressures at two points in a pipe are measured. This method was applied to measuring the outgassing rates of two kinds of SUS304 plates, belt‐polished plates, and buff‐polished plates. The outgassing rates were measured under a pressure as low as that expected in the actual high‐vacuum system. Additionally, the one‐point pressure method has been introduced, whose validity was ascertained using measured pressures in the experimental setup. The outgassing rates for the same kinds of SUS304 plates were again measured by the conventional orifice method for comparison.

A cascade diffusion pump system for an electron microscope

Several DP systems have been discussed and evaluated with respect to dynamic evacuation under actual operating conditions of an electron microscope. As a result, a cascade DP system has been found to be the most reasonable for an electron microscope where the column is evacuated by the first DP backed by the second DP, and for the camera chamber by the second DP with a large volume foreline backed by a mechanical pump. A spectrum of the residual gases in an electron microscope with a cascade DP system indicates a very clean vacuum with hydrocarbon partial pressures of less than 3×10−7 Pa.

Mechanism of contamination build-up induced by fine electron probe irradiation

Abstract To clarify the transfer mechanism of contaminating molecules and the dominant source of them, carbon thin films were irradiated with a fine electron probe of 12 nm dia using three types of anti-contamination devices, individually. No contamination deposit was observed for a certain period or an incubation time after the specimen was pretreated by an electron beam shower. After the incubation time, the growth rate increased gradually with the elapsed time after the electron beam shower treatment and reached the saturated rate. The incubation time and the saturated growth rate depended largely on the type of the anti-contamination device used. It can be concluded that hydrocarbon molecules being adsorbed on the specimen surface are transferred to the irradiated area by surface diffusion. The dominant source of contaminating molecules is the residual gas molecules in the vicinity of the specimen even in a clean high vacuum.